A number of techniques were proposed in the art for imparting water repellency to the surface of solid articles such as plastic articles. One typical attempt is to effect plasma treatment in vacuum with fluorides such as CF.sub.4, C.sub.2 F.sub.6, SF.sub.6 and NF.sub.3. The plasma treatment causes fluorine atoms and fluorinated groups to non-selectively bond with valence bonds available on the article surface to cover the article surface with the fluorine atoms and fluorinated groups, resulting in the article surface having a molecular structure analogous to polytetrafluoroethylene (Teflon) or a structure having fluorinated groups as typified by CF.sub.3 groups. Consequently, the articles have so reduced surface energy that the surface becomes water repellent.
The plasma treatment with fluorides such as CF.sub.4, C.sub.2 F.sub.6, SF.sub.6 and NF.sub.3, however, achieves water repellency to a less satisfactory extent in a manner that water repellency diminishes within a relatively short time. Since the plasma treatment of this type generally requires a vacuum of about 10 Torr or lower, a vacuum apparatus of a large size is necessary for the commercial practice of plasma treatment, adding to installation investment and operation cost. If articles to be treated have relatively high contents of water and/or volatile plasticizers, these components will vaporize in a vacuum atmosphere to leave the article surface, sometimes interfering with the plasma treatment in imparting the desired function or property. Further, the plasma treatment of this type is difficult to apply to articles of low-melting materials because heat is often generated during the treatment.
Another attempt for imparting water repellency to articles is proposed by Japanese Patent Application Kokai No. 235339/1987 wherein the articles at the surface are subjected to glow discharge plasma treatment under atmospheric pressure with a mixture of a fluorinated compound such as CF.sub.4, C.sub.2 F.sub.6, SF.sub.6 and NF.sub.3 and an inert gas at atmospheric pressure, thereby reducing the surface energy of the article. This method achieves more lasting water repellency as compared with the plasma treatment of the aforementioned type. It does not need a large size vacuum apparatus because the treating pressure is approximate to atmospheric pressure. In addition, it is fully applicable to articles having relatively high contents of water and volatile plasticizers as well as to articles of low-melting materials because little heat is generated during treatment. However, the level of water repellency achieved by this treatment method is still unsatisfactory like the aforementioned vacuum plasma treatment. There is a need for improving water repellency.
On the other hand, some inorganic materials such as metals, semiconductors and metal oxides are difficult to directly treat with fluorides for imparting water repellency. To impart water repellency to such blank materials, Japanese Patent Application Kokai No. 238961/1986 discloses a method including the steps of first subjecting an article to glow discharge plasma treatment under atmospheric pressure with a mixture of a hydrocarbon gas and an inert gas to deposit a film of carbon or hydrocarbon on the article surface and then treating the coating for imparting water repellency. Unfortunately, the carbon or hydrocarbon film is deposited at a very slow rate of approximately 100 .ANG./min. In addition, this method is cumbersome because of the two-step process including film deposition and water repellency imparting steps. There is a need for a treatment method capable of imparting water repellency to even these inorganic materials.